Cameras are used in a large variety of applications in order to capture and record still pictures and video streams. In order to capture or record an image or a video stream, light originating from the scene to be captured is collected using some form of lens system of the camera in question. The light originating from the scene is then focused by means of the lens system and the focused light is directed onto an image sensor, which in turn records the incident light originating from the scene. The image sensor is generally built up of several pixels. When the light from the scene as focused by the lens system reaches the image sensor and is focused onto the same, image data can be recorded for each pixel of the image sensor, resulting in a pixel level. It is important that the incident light is focused when reaching the sensor in order to be able to acquire a sharp and not blurry image. The pixel level as recorded for each of the pixels is then in turn used to represent the scene being captured by the camera.
Generally a camera is employed with some form of aperture or iris to control the amount of light and its characteristics entering the camera housing and consecutively reaching the image sensor of the camera in question. The ability to be able to control the amount of light reaching the sensor is generally essential, as too much light reaching the sensor results in an over exposed image carrying less information of the scene being captured. Correspondingly it is also important to be able to collect as much light as possible when recording a dark scene. Also the depth of field is affected by the aperture setting.
Conventionally the size of the aperture, and consequently the amount and the characteristics of the light reaching the image sensor, is controlled by means of an iris often referred to as a conventional iris. The conventional iris or iris diaphragm is conventionally made of number of thin metal plates which are mounted in a circular manner next to each other to form a relatively circular centered aperture. The size of the aperture can be adjusted by changing the relative angles of the metal plates. When changing the relative angles, the size of the aperture is changed although the circular shape and its central location is in principle kept constant.
When using a conventional iris or aperture, of the type described above, when capturing an image, certain light conditions may result in that unwanted light is recorded by the image sensor of the camera. The unwanted light will result in an image having a lower image quality. A particular form of unwanted light is referred to as stray light or parasitic rays. By stray light is generally meant light or light components not belonging to an image of a scene but that rather has been added to the image in question, added in the widest possible sense. In practice the stray light may originate from the scene or may not originate from the scene. In certain conditions the negative effects of stray light are significant. For instance when a lens assembly having a conventional iris is used under strong lighting conditions, the negative effects from the stray light become clearly disturbing. Just to give a few examples, the sun or another strong light source may lead to ghost images of the light source in question. The ghost images which can significantly reduce the image quality of the acquired image are representations of the aperture.
Further the edges of the metal plates of the iris may lead to diffraction resulting in an image having a lower image quality.
Another phenomenon leading to that stray light is reaching the image sensor is internal reflections in the optical components of the lens assembly or the camera used. When a surface of an optical component exhibits a defect, light may be directed into the glass of the optical component and then be transferred within the component to a different location. This will result in that small defects in e.g. a lens may lead to that stray light is reaching the image sensor which in turn results in an image having a lower image quality.
Hence, there is a need for an improved method and lens assembly for reducing stray light when acquiring images or video streams.